let p1 = new Promise((resolve, reject) => {
  resolve(Promise.resolve('success'))
}).then(res => {
  console.log(res) // success
})


let p2 = new Promise((resolve, reject) => {
  resolve(new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('123456')
    }, 3000)
    resolve('789')
  }))
}).then(res => {
  console.log(res) // 789
})